The present invention relates to photochromic articles comprising a substrate having at least a partial coating of an at least partially cured coating of a photochromic polymerizable composition. Photochromic compounds can be added to the composition prior to polymerization or after an at least partially cured coating is formed. The present invention also relates to photochromic articles that are optical elements having at least a partial coating of an at least partially cured coating of a photochromic polymerizable composition.
Photochromism is a phenomenon involving a reversible change in color of a photochromic compound, or an article containing such a compound, upon exposure to light radiation containing ultraviolet rays, and a reversion to the original color when the influence of the ultraviolet radiation is discontinued. Sources of light radiation that contain ultraviolet rays include, for example, sunlight and the light of a mercury lamp. Discontinuation of the ultraviolet radiation can be achieved for example by storing the photochromic compound or article in the dark or by removing the source of ultraviolet radiation (e.g., by means of filtering).
The general mechanism responsible for the reversible change in color, i.e., a change in the absorption spectrum in the visible range of light (400-700 nm), exhibited by different types of photochromic compounds has been described and categorized. See John C. Crano, “Chromogenic Materials (Photochromic)”, Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, 1993, pp. 321-332. The general mechanism for the most common classes of photochromic compounds, e.g., indolino spiropyrans and indolino spirooxazines, involves an electrocyclic mechanism. When exposed to activating radiation, these compounds transform from a colorless closed ring compound into a colored open ring species. In contrast, the colored form of fulgide photochromic compounds is produced by an electrocyclic mechanism involving the transformation of a colorless open ring form into a colored closed ring form.
In the aforedescribed electrocyclic mechanisms, the photochromic compounds require an environment in which they can reversibly transform. In solid polymer matrices, the rates at which the photochromic processes of activation, i.e., formation of color or darkening, and fading, i.e., the return to the original or colorless state, occur are believed to be dependent on the free volume in the polymer matrix. The free volume of the polymer matrix is dependent upon the flexibility of the chain segments of the polymer environment surrounding the photochromic compound, i.e., the local mobility or local viscosity of the chain segments comprising the matrix. See Claus D. Eisenbach, “New Aspects of Photochromism in Bulk Polymers”, Photographic Science and Engineering, 1979, pp. 183-190. One of the main obstacles reported by Claus D. Eisenbach, for the larger commercial application of photochromic systems, is the slow rate of photochromic activation and fade in a solid polymer matrix.
Recently, photochromic plastic materials have been the subject of considerable attention in part due to the weight advantage that they can offer relative to ophthalmic lenses made of glass. In addition, photochromic transparencies for vehicles, such as automobiles and airplanes, have been of interest because of the potential safety features that such transparencies offer.
In addition to the slow rate of activation and fade of photochromic compounds in polymer matrices, a further drawback to the widespread commercial use of organic photochromic compounds in combination with plastic materials is the loss of their ability to exhibit a reversible change in color as a result of prolonged repeated exposure to ultraviolet (UV) light. This phenomenon is believed to be a result of irreversible decomposition of the organic photochromic compound and is referred to as fatigue.
While some advancements have been made toward increasing the fatigue resistance and improving the performance of photochromic materials, further improvements, even small incremental improvements, in the fatigue resistance and/or improving the performance of photochromic polymeric materials are still required. Hence, efforts to obtain such improvements have continued.
Although the use of photochromic compounds in combination with polymerizable compositions, e.g., (meth)acrylates, is known, the use of the polymerizable composition of the present invention with photochromic compounds as at least a partial coating of an at least partially cured coating on a substrate has not been disclosed.